1. Field of the Invention
The present invention relates to a braking systems, and more particularly to a brake pressure control device which is able to control a brake pressure by means of an air control valve for anti-lock control.
2. Discussion of the Related Art
Conventionally within the field of braking systems, brake pressure control devices that execute a braking operation by means of air pressure are known. Examples of which are disclosed, for example, in Japanese Patent Publications Nos. 2-256555 of Heisei and 8-58546 of Heisei.
The brake pressure control device disclosed in the above-mentioned Japanese Patent Publication No. 2-256555 of Heisei is an anti-lock brake device which includes a modulator valve connected between a brake valve and a brake device for converting the low pressure of compressed air pressure fed from the brake valve to a high oil pressure necessary to brake a wheel by use of such oil pressure. The modulator valve is utilized to selectively bring the compressed air provided from the brake valve into communication with the brake device or selectively reduce the pressure of the thus communicated compressed air, and such selective communication and selective pressure reduction within the modulator valve is achieved by way of a port formed in a pipe passage communicating between the brake valve to the brake device. A valve energized toward the port by a spring is provided, and the valve is allowed to selectively open or close the port. The modulator valve also includes a check valve which is arranged in the pipe passage to prevent the air pressure from flowing into the pipe passage portions in the front of and in the rear of the above-mentioned energized valve from the upstream side to the downstream side.
Due to this structure, this anti-lock brake pressure control device is able to relieve or eliminate brake dragging after a brake pedal returns to its resting position after being activated.
However, when this anti-lock brake device is switched from its brake operation condition to its brake release condition, the air pressure on the air master cylinder side is directed through hold diaphragms within an air control valve and then through the air control valve itself. The air pressure is then directed through a long pipe passage connecting or communicating the air control valve to the brake valve, and is finally discharged to the open air through an exhaust port formed in the brake valve. Thus, the passage for releasing the air pressure in the brake opening operation is long, which, together with fluid frictional resistance and the like produced within the pipe passage, makes it difficult to reduce the air pressure of the air master cylinder (to release the brake) quickly.
In other words, since the pipe passage for releasing the air pressure in the brake releasing operation is so long, any improvement in the response performance of the brake device is extremely limited.
Moreover, in this conventional brake device, when the air master cylinder is in operation, the open air must be introduced into or discharged from a piston back pressure chamber formed within the air master cylinder and, as a result, the use of an air breather is required. This air breather must be structured such that it has an environmental resistance in order to prevent water or dust from flowing towards the piston back pressure chamber. This increases the cost of the air breather. Also, the breather installation requirement increases the number of man-hours required to assemble the braking system, thus increasing the production costs and operation complexity of the braking system.
On the other hand, the brake pressure control device disclosed in the above-mentioned Japanese Patent Publication No. 8-58546 of Heisei comprises a brake control valve which is interposed between a compressed air source for generating a braking operation pressure using compressed air and a brake actuator. The braking operation pressure is supplied for generating a brake pressure for braking a wheel and to control the supply and discharge of the brake operation pressure with respect to the brake actuator. A modulator including an electromagnetic valve is interposed between the brake control valve and the brake actuator. This modulator controls the brake operation pressure to be supplied to the brake actuator.
In the brake pressure control devise, the modulator includes a pressure holding diaphragm for holding the brake pressure in an anti-lock control operation, and a pressure discharging diaphragm for discharging the brake pressure, whereby, as the need arises during its anti-skid brake control operation, the diaphragms can be opened and closed to thereby be able to prevent the brake from being locked.
However, in the above-structured brake pressure control device, the modulator (also known as an air control valve) must always require two diaphragms for holding and decaying or discharging the pressure, which, together with a passage for allowing the two diaphragms to communicate with each other, makes the whole structure of the modulator relatively complex. Moreover, the use of the two diaphragms increases the component count of the braking system. That is, there is a need in the art of brake pressure control devices to be able to reduce the size, complexity, and weight of these devices.
In particular, as the number of passages formed within an air control valve housing increases, the number of seal surfaces increases accordingly, which both makes the housing larger in size as well as increases the time necessary for assembling the housing. Moreover, as the number of diaphragms increases, the complexity of the system increases and the time necessary to assemble the diaphragms to the housing also increases.